The relationship between stress and strain of powders or granular materials can be difficult to determine. Unlike a specimen of solid material, a specimen of powder cannot be easily placed under uniaxial stress and examined. For example, for testing steel, a small piece of steel can be used as a specimen, placed under uniaxial stress, whereupon the resulting strain can be measured.
Because of their inherent nature, powders and granular materials must first be loaded in all three directions (triaxially) in an initial hydrostatic (pressure all around) condition. This permits the load on one of the axes to be increased to obtain the strain response of the loaded axis.
One approach to loading a specimen of powder triaxially is to provide a specimen having a right circular cylinder geometry. The specimen is subjected to constant confining pressure, using hydraulic pressure in the radial direction while varying the load in the long axis with mechanical means. This allows measurement of strain in the long axis, but measurement of strain in the radial direction becomes challenging, as strains in that direction can be upwards of 50%. A common strain gage is inadequate for this type of measurement.
For powders, properties such as the relationship between stress and strain or the Poisson's ratio, are referred to as “constitutive” properties. Once known, these properties can be used to predict the behavior of powders for diverse applications.
To determine constitutive properties, a small specimen of powder (or other granular material) can be evaluated, and its measured properties can then be used to predict the behavior of large amounts of the powder. The measurements can be used in simulations and other calculations. For example, if the properties of a sand specimen were to be measured, the penetration of a ballistic projectile into sandy terrain could then be simulated. If, in another example, measurements were made on an asteroid's response to a human-induced impact event, the results could be compared with simulations of the event using material characteristics of a catalog of powders and granular materials to verify the asteroid's composition.